Analog world is an exciting field having great potential for commercial exploitation. Most of the things do occur in nature in analog way and the processing the signal in analog domain is the natural way of signal processing. This processing does not require conversion to any other domain and hence is natural and fast. Contrary to this, the processing of the signal in digital domain requires signal conversion into digital domain from analog domain and then back conversion into analog mode after processing. The processing in digital domain may be fast enough but the processing time is actually controlled by the devices used for analog to digital and digital to analog conversion. The time used by the processor is actually too low. This also increases the hardware count for the signal processing and hence, such a system (digital signal processing) is more complex which in turn returns to slow signal processing.
The analog signal processing however is a high frequency operation, which is being further, enhanced by the use of current mode signal processing elements. There is a need to have constant development and perennial work in this field to exploit its full potential. Current conveyors (CC) are most powerful current mode signal processing block gaining importance as a practical and high performing circuit structure for future analog signal processing applications. Earlier versions of CC use conventional op amps for construction. However, CCs are available in monolithic form today. Many workers have worked towards this end and proposed bipolar/CMOS implementation of the CCII structure. Even BiCMOS implementations are also being proposed. Almost all of them operate at ±3.0 V or higher. To our knowledge only one CC is proposed which is capable of operating at ±1.0 V, but that too have lower bandwidth (<30 MHz). Moreover, the circuit structure is too complicated. The input voltage buffer used is too complicated as it uses many CMs and many current summing nodes.
Current conveyors (CC) represents the emerging class of high performing analog circuit structures, based on current mode approach in circuit design. In a current mode approach, an analog designer considers the currents as input and output variables. Thus, a current mode device is defined as the circuit structure whose all functions can be fully explained and under stood through the current flowing into its various sub-circuits without considering the input and output voltages at all. However, appropriate bias voltages must establish the proper operating conditions in circuit structures. The merits of current mode circuits include available wide frequency bandwidth, their capability to operate at low voltages and simple circuit structures. Low voltage operation is a the most favorable design technique for getting low power circuit structures, which, however, may not translate into low power circuits.
M/S Analog Devices USA has come up with few current conveyor II architectures in the form of current feedback amplifiers. AD 844 is the most popular current conveyor, which require minimum of ±3.0 power supply voltages and have the bandwidth of 10 MHz. Moreover the device has been implemented using bipolar technology. Several other chips have also been fabricated and are available commercially. Though these commercially available chips offer high bandwidth, still they suffer on account of high power consumption and need for higher bias voltages. Almost all of them are implemented in bipolar technology.
Analog designers are now concentrating on the design of low power and necessarily low voltage circuits to cope up with the demands of portable instruments and mobile communication gadgets. Low power circuits are preferred in non-portable instruments also. The merits of a current conveyor encourage one to use them in power miser, low voltage and high performing current mode circuits. They are becoming the industry standard for high frequency applications. The proposed CC structure is another major step forward in this direction.
Current conveyors are very versatile analog signal processing blocks and are now replacing the conventional operational amplifiers in most of the signal processing applications. Some of the applications of CCs are:
Analog Active Filters
This is an important class of applications for CCs. Analog filters find many uses, which include                i. Entertainment electronics        ii. Control circuits for highly noisy industrial environment. This type of environment is also present during the launch of a spacecraft and the circuits have to perform up to the mark.        iii. In hand held portable instruments and communication gadgets.Space Application        
When the designer intends to design the instruments for space exploration, he faces the challenges on four equally important fronts viz., power consumption, size, weight and reliability. Hence there arises a need to have reliable low power high performing circuits. Thus the CCII may prove to be a boon in the design of such low voltage low power scientific instruments for space exploration and strategic military applications.
Medical Electronics
Medical aids are increasingly being employed as life saving aids. Particularly, instruments like pace makers and hearing aids are being used increasingly. The requirements of medical aids are small size and low power consumption. CCII can be used successfully in the construction of these medical aids.
General Instruments
General-purpose instruments operating at higher voltages and consuming high power require cooling system in form of a fan or a water-cooling arrangement. This increases the instrument weight and reduces the portability and thus the optimum utilization of the instrument. The low voltage low power CCII can be used in the design of such measuring instruments to increase the portability.
It may be noted that the potential applications of CCIIs lie in the high performance like high bandwidth and low power consumption (low voltage operation).
A CC as shown in FIG. 1 is a three (or more) port network, which are designated as port X, port Y and port Z. Port X is a dual port suitable for input as well as output signals. For current signals, it acts as a input port with very low input impedance, but is an output port for voltage signals. Port Y is only voltage-input port with high input impedance. Port Z is a current output port. For the injected current into port X, the output port Z may sink or source the current. Further investigations into CCs led to the emergence of few new circuit architectures. Thus there are two types of the classifications of the CCs, one of which is based on the input port Y characteristics and the other on the port Z characteristics. Based on the input port Y characteristics, CCs have been classified as CC type I (CCI), CC type II (CCII), and CC type III (CCIII). When classification is based on port Z, the CCs are classified into CC+ and CC−. The port properties of a general CC structure are given as:                               [                                                                      i                  Y                                                                                                      V                  X                                                                                                      i                  Z                                                              ]                ≡                              [                                                            0                                                  A                                                  0                                                                              1                                                  0                                                  0                                                                              0                                                  B                                                  0                                                      ]                    ⁡                      [                                                                                V                    Y                                                                                                                    i                    X                                                                                                                    V                    Z                                                                        ]                                              (        1        )            
For CCI, A=1 and a voltage VY will be connected at port Y and voltage at port X, (VX) follows VY independent of current injected into port X. The voltage VZ produced at port Z is arbitrary. In CCII, A=0 and the current into port Y is zero, offering very high input impedance at port Y. For CCIII, A=−1, which implies that IY=−IX. This property is utilized in monitoring the current in circuit paths. For CC+ structure, B=1, while for CC− structure, B=−1. Thus CCs form nine subcategories. These are CCI+, CCI−, CCI composite outputs, CCII+ CCII−, CCII with composite outputs, CCIII+, CCIII−, and CCIII with composite outputs.
Elements of a Current Conveyor
Current mirror (CM) is an integral part of almost all-analog circuit architectures. Hence when we talk of low voltage circuits than it is necessary to have their sub circuits which are capable of operating at low voltages. To meet this challenge there is need to design high performing CMs which are capable of operating at low voltages. These Low voltage current mirrors (LVCMs) are used in the design of proposed CCII structure. The LVCM is depicted in FIG. 2.
The performance indexes of a LVCM on which the performance of a CM can be assessed are:
i. Low input impedance ideally zero,
ii. High output impedance ideally infinite
iii. High current transfer bandwidth ideally infinite
iv. High dc current transfer range ideally infinite
v. Rail to rail input and output voltage swing capability.
Most of the analog circuit structures are hybrid in nature, which utilize both current and voltage mode signal processing elements. Operational amplifier is one such example, which uses both, the current mode circuits such as current mirrors, and a differential pair, which is based on voltage mode concept. Thus, for the completeness of a the any current mode circuit, it is essential to examine few low-voltage analog-voltage mode circuits also which are suitable for being used in current mode devices. Their applications into the low voltage signal processing analog cells demand a fresh look.
Voltage Buffer, one of the extremely powerful circuits and most popularly used to transfer the input voltage, impressed at a port normally known as input port, to the other port (normally termed as output port), possessing the high current sourcing/sinking capabilities. The properties of these analog circuits include:                Accurate voltage tracking, necessary to get exact voltage transfer without any error.        Low output impedance, required to increase the current sinking/sourcing capability of these circuits.        High input impedance decreases the input signal loading.        High dynamic range provides larger signal range for input voltage signals.        Low power dissipation gives high battery life and smaller size.        
These properties facilitate the analog designer to use these blocks for isolation between the input port and output port. High input impedance is necessary to avoid the loading of the input signals. They also provide high current drive capability to the output port. Hence they can also be termed as voltage buffer. These blocks find wide-ranging uses. Most common among these uses which are suitable for current mode signal processing analog cells are their use in current conveyors, current feedback amplifiers, operational floating amplifiers etc. They are employed at input port of a current conveyor for transferring the voltage at the current input port. When it is used at the output port of a CCII, the resultant circuit structure acts as a current feedback amplifier, generally a high performance device. Four terminal floating nullers (FTFNs) are the most popular current mode circuits, which are suitable for being used as a general purpose current mode block. This block is used in FTFNs to form its voltage-input port. Even the voltage mode circuits uses this block very successfully in operational amplifiers. It is placed at the input end of operational amplifiers.
Voltage transfer blocks were implemented using several circuit structures. Almost all implementations use a differential pair. Differential pair is one of the very important analog cells, which is used in many circuit applications originating due to real word problems. Design of low voltage rail to rail voltage transfer cells is very important, as they require further investigation into various new circuit structures. They are called operational trans-conductors also. They are used to transfer the input voltage signals from a port to another low impedance port.
Several circuit schematics are available in the literature, for being used as voltage transfer block. However, all these blocks are operated at relatively higher supply voltages. Their input and output voltage swings are also limited. To overcome these deficiencies, several circuit structures are proposed, which are capable of providing rail to rail voltage transfer. However, a few structures are suitable to operate at low voltage levels. Thus examination of new circuit structures capable of operating at low voltage and providing rail to rail voltage transfer capability, is an extremely important task.
Source coupled pairs, most commonly used to form a differential input stage. It is most popular two MOSFET sub circuits in monolithic analog circuit structures. The usefulness this circuit stems from the fact that cascades of this source coupled MOSFETs are directly coupled to one another without inter stage coupling capacitor and that differential input characteristics are required in many types of analog circuits.
An important objective of the differential amplifier design is minimization of the dc bias current flowing into the input leads of the circuits and maximization of differential input resistance. MOSFETs are preferred device for such a configuration to have low input bias currents.